Despite significant progress in cancer treatments, a number of malignant tumors remain deadly diseases. Among those are central nervous system (CNS) tumors, which are the leading cause of cancer death for people under 35 years of age. The incidence of primary brain tumors increased by more than 25% between 1979-1991 and the death rate increased by 15%. Finding novel treatments for brain tumors is currently a major challenge, especially for malignant gliomas, which have the highest death rates (>13,000 deaths/yr in the United States). Patients with GBM have an average survival of 10-12 months and a 2-yr survival rate of less than 10%, irrespective of therapy.
Cancer can be a fatal disease, in part, because cancer can spread or metastasize throughout an organism. Metastasis plays a major role in the morbidity and mortality of breast cancer. Breast cancer metastasizes in a stereotypical pattern resulting in lesions found in the lymph node, lung, liver, and bone marrow. Generally, cancer cells lose differentiated properties, proper tissue compartmetalization, cell-cell attachment as well as obtain altered cell substratum attachment, altered cytoskeletal organization, cell locomotion, and the ability to survive at distant sites.
Hypoxia is a major hindrance to effective solid tumor therapy. The microenvironment of rapidly growing solid tumors is associated with increased energy demand and diminished vascular supply, resulting in focal areas of prominent hypoxia, regions with reduced oxygen tensions. Tissue oxygen electrode measurements taken in cancer patients showed a median range of oxygen partial pressure of 10 to 30 mmHg, with a significant proportion of readings below 2.5 mmHg, whereas those in normal tissues ranged from 24 to 66 mg. In the absence of oxygen, which is the most electron-affinic molecule in cells and reacts chemically with the fundamental biological lesion produced by ionizing radiation, radiotherapy is severely compromised in its ability to kill hypoxic tumor cells. On the other hand, hypoxia (and possibly hypoxia-associated deficiencies in other nutrients such as glucose) causes tumor cells to stop or slow their rate of progression through the cell cycle. Because most anticancer drugs are more effective against rapidly proliferating cells than slowly or non-proliferating cells, this slowing of cell proliferation leads to decreased cell killing. Chemotherapy is further affected by hypoxia as chemotherapeutic drugs are delivered systemically and the diffusion of these into the tumor makes the hypoxic regions exposed to less drug than the oxygenated cells proximal to the vessels. Moreover, the multidrug resistance (MDR1) gene product P-glycoprotein is induced by ambient hypoxia.
Tumor hypoxia increases malignant progression and metastasis by promoting angiogenesis through induction of both pro-angiogenic proteins such as VEGF and metabolic adaptation through elevation of glycolytic enzymes. Hypoxia also generates selective pressure for cells to acquire genetic alterations (e.g., TP53, K-ras), that will circumvent hypoxia-induced apoptosis.
An essential component of tumor growth is angiogenesis. Tumors need to disrupt physiological controls over angiostasis to initiate neovascularization, a process triggered by the release of hypoxia-inducible angiogenic factors by nascent tumors. Angiogenesis is a stepwise process during the grade II-IV progression of astrocytoma. First, new blood vessels appear in low grade astrocytoma (II) followed by an increase in vessel density in anaplastic astrocytoma (III). Then, with the transition to GBM (IV), extensive micro-vascular proliferation leading to abnormal vessels occurs. Hypoxia is an integral component of astrocytoma progression and increases with grade. Most PO2 readings are in the 0.5-2.5% range, although severe hypoxia (0.1% range) has also been reported. Hypoxia-mediated angiogenesis is most prevalent in the transition from grade III to IV tumors. Hypoxia occurs at the leading/actively growing edge of tumors where it leads to the florid microvascular proliferation characteristic of GBM. The combination of hypoxia/micro-vascular proliferation accelerates peripheral expansion of GBM up to 10-fold compared to lower grade astrocytoma, while over time the center of the tumor becomes anoxic and necrotic. Despite their appearance on MR imaging, GBM are not like spheroids with central hypoxia and necrosis. Hypoxia occurs over micron distances as do changes in oxygen gradients. Pimonidazole staining for hypoxic regions in the actively growing part of experimental and human GBM shows micro-constellations of hypoxic regions. Immunohistochemistry studies in human GBM have also shown that these regions strongly stain for HIF-1, a major regulator of the physiologic response to hypoxia. The appearance of hypoxia is a critical physiological change that heralds a more malignant tumor behavior which dramatically reduces patient survival. Vascularity and microvascular cell proliferation are morphological features used to distinguish grade IV gliomas from grade II/III and they correlate with patient prognosis. Angiogenesis is known to occur as the result of a disruption in the balanced synthesis of molecules that stimulate and inhibit new blood vessel formation. VEGF, the most important known regulator of tumor angiogenesis is transcriptionally upregulated by HIF-1. In situ hybridization has shown that VEGF mRNA is strongly expressed in pseudopalisading cells, a rim of viable hypoxic tumor cells that line micro-necrotic areas in GBM and which express high levels of HIF-1. In addition to promoting angiogenesis, hypoxic tumor cells are also refractive to radio- and chemo-therapies. Therefore, hypoxic areas of astrocytic tumors represent an important target for anti-tumor therapy and preliminary clinical studies targeting hypoxia have shown modification of outcome in GBM.
HIF is the primary transcription factor activated by hypoxia. Its activation and regulation are complex, with numerous points of potential inhibition. Active HIF is composed of alpha (HIF-1α, 2α) and beta (HIF-1β) subunits that dimerize and bind to consensus sequences (hypoxia responsive elements, HRE) in the regulatory regions of target genes. HIF controls the expression of more than 60 target genes whose products are critical to many aspects of tumor progression, including metabolic adaptation, apoptosis resistance, angiogenesis and metastasis. These include VEGF, erythropoietin, glucose transporters, and glycolytic enzymes. In normoxia, HIF is hydroxylated and interacts with the von Hippel Lindau protein (pVHL), an E3 ubiquitin ligase subunit that targets HIF for degradation. In the absence of oxygen, HIF hydroxylation is inhibited, preventing binding to pVHL and leading to its intracellular accumulation. HIF-1 has been recognized as an important molecular target for solid tumor therapy due to its crucial role in tumor angiogenesis and progression. Increased levels of intracellular HIF-1α are found in many cancers and are associated with poor prognosis and resistance therapy. HIF-2α upregulation is found predominantly in cancers with VHL gene mutations. HIF-1α expression correlates with tumor grade and vascularization in gliomas, while HIF-2α expression is usually absent. The relative importance of HIF-1α and HIF-2α subunits in different tissues and cancer types is still under investigation as are their multiple levels of regulation.
Accordingly, there is a need for new and effective treatments for cancer. In particular, there is a need for new and effective treatments that address hypoxia and its role in hyperproliferative pathologies.